Some of the questions you ask don't have a single right answer as much as preferences. Some of my thoughts and experience:
1. West Coast vs. East Coast. I've cycled both. I liked the West Coast better and the winds go the right direction. However, beginning of October starts to be in hit/miss season on the Pacific Northwest. Also you have to get there (?) from AB so there is potential of early storms in the rockies. You could get lucky and not have too much rain or could just be at the start of the more rainy season.

2. Choice of bike/trailer/panniers. I've done most of my touring on a touring bike including a ride across Russia mostly on pavement but with almost 1000 miles of gravel road. So a touring bike can handle the rough roads, particularly if you put slightly wider tires on it during the roughest places. At the same time, I did a supported ride across Africa this year and chose a Trek 4500 mountain bike. I fitted it with some extra bar end handlebars for more hand positions and it had a front suspension. When I got back from Africa, I landed in Portland OR and rode another 1940 miles of self-supported touring with the bike. So I've done both types of bike for touring and if you put narrower tires on your rockhopper and make sure it will be comfortable many hours in the saddle/handlebars than you could likely make that work too. On the flip side, if you have a touring bike some slightly wider tires can make it handle some rough roads. Probably more important to make sure whatever bike you pick will be comfortable.

3, Panniers vs. trailer. Again most of my cycling has been touring bike with panniers. That is comfortable and works well for me. However, when I did the touring this year on my mountain bike, I picked up an extrawheel trailer (photo of my rig: http://www.bike2013.com/wp-content/g...e/dsc_0459.jpg). I liked the extrawheel trailer because it attached low on the wheel as opposed to up higher on the seat post. Hence, it tracks well and was less noticeable than when I ride with a Bob trailer. Again, either panniers or trailer could be made to work.